1. Field of the Invention
The invention concerns a recording paper with an internal size and a reverse side surface preparation, whose front side is intended for receiving recordings, and whose reverse side has a different structure from the front side and can be printed with a printing ink that contains functional pigments. The invention also concerns the use of the recording paper of the invention.
2. Description of the Invention
There are many different kinds of paper or cardboard substrates that are intended for recording, especially substrates of this type whose front side has a coating or preparation intended for certain recording processes and whose reverse side has a different coating or preparation from the front side.
EP 0 120 095 A1 describes a cast coating process, in which water is applied to the reverse side in amounts of 1-30 g/m2 by jets or screen rolls to prevent curling in the coating machine. The moistening water may also contain starch, protein, synthetic sizing agents, wax emulsions, wetting agents and pigments.
DE 3 621 732 A1 describes a process, in which, to prevent curling during the production of paper or cardboard that is treated on one side, especially paper that is coated on one side, the untreated or uncoated side is passed through a water tank, so that the paper web absorbs a sufficient amount of moisture over the entire width of the web, and is then dried as usual.
EP 0 402 041 describes a heat-sensitive recording material, in which wrinkling of the paper sheet to be used as the substrate is to be reduced by selecting certain types of pulp. The paper sheet preferably contains the usual papermaking additives, e.g., strength additives, such as cationic starch, sizing agents, such as cationic and anionic polyacrylamides or salts of fatty acids, sizes based on AKD or on an alkenyl succinic acid, agents for improving wet-web strength, and inorganic fillers. Preferably, the paper sheet is also impregnated with aqueous solutions of polymers applied by a size press, such as polyvinyl alcohol, starch, SBR latex, or styrene-maleic anhydride copolymer.
The prior art discussed below includes several proposals which seek to improve recording quality by making available a substrate with a certain smoothness and a certain water-absorption capacity. In this regard, these previous proposals differ considerably in the proposed values for smoothness and water-absorption capacity.
With reference to JP-A-08072394, Patent Abstract of Japan describes a recording paper that is produced from a substrate, one of whose surfaces has a Cobb value in the range of 15-50 g/m2 and an Oken smoothness value of 40-400 s. The smoothness and Cobb value of the finished recording paper are not disclosed.
GB 2 163 271 A also proposes a special substrate for obtaining a heat-sensitive recording paper with improved recording density and superior point reproducibility, which is intended especially to make possible a high printing speed of the recording paper in a thermal printer. In the case of this previously known substrate, an interlaminar strength of 0.5 to 2.0 kg cm by TAPPI UM 528 and a Cobb value of 10-20 g/m2 by JIS P 81440 are adjusted by the selection of the pulps and by specific addition of surface sizing agents and other additives. The substrate is supposed to have a Bekk smoothness in accordance with JIS P 8119 of at least 200 s. According to statements in the patent specification, both the Cobb and Bekk values refer to the front surface, on which the heat-sensitive recording coating is applied. As the tables in this document show, good results are achieved with substrates with a Bekk smoothness of 254-402 s and a Cobb value of 14.9 to 18 g/m2. Since it can be inferred from the description of the example that application of the heat-sensitive recording coating is followed by a calendering step, it may be assumed that the smoothness of the finishing recording papers is even higher than the values disclosed for the various substrates. Coating of the reverse side of the substrate with a latex dispersion or water-soluble polymers is designed to prevent wrinkling of the substrate. Cobb or smoothness values are not disclosed for the surface on the opposite side from the recording coating either for the substrates provided with a heat-sensitive recording coating or for substrates without a recording coating.
EP 0 887 199 A2 proposes a printing material for ink-jet printing, in which the paper substrate is supposed to have a density of 1.01 g/cm3 or more and a degree of sizing (determined as the Cobb value) of 1-15 g/m2, and preferably 2-11 g/m2. The goal of this proposal is to minimize the wrinkling of the recording material during printing with aqueous ink by making available a substrate with low unevenness for the production of recording materials with improved image definition. Even though smoothness values for the substrate and the finished recording material are not disclosed, it may be assumed that the material has a relatively high smoothness due to several calendering operations, to which both the substrate and the recording paper that has been provided with a recording coating are subjected.
EP 0 190 875 A2 describes a recording material in which a heat-sensitive recording coating is applied to a surface-sized paper substrate that has a Cobb value of 25 g/m2 or less, preferably 10 to 20 g/m2*, as measured by JIS P 8140, which corresponds to DIN/EN 20535 or ISO 535. This proposal aims to achieve good contact with the thermal printing head to improve the recording quality.
EP 0 900 668 A2 provides for a substrate with a Cobb60 value of 30-100 g/m2 in accordance with JIS P 8140 for a recording material for ink-jet printing, which is to be produced by the cast-coating process. The substrate is supposed to have a smoothness of at least 30 s as measured by JAPAN TAPPI No. 5, and values of up to 60 s are disclosed as examples. Increased productivity in the production of cast-coated papers and improved ink absorption are strived for with the substrate.
Finally, DE 195 32 303 C2 discloses a recording material for ink-jet printing, in which a support material based on a synthetic thermoplastic polymer is used, which has a Cobb12 value of at least 30 g/m2 in accordance with ISO 535 (E1976). The recording material produced in this way is intended to overcome the stated disadvantages of long-lasting, complete absorption of applied ink within the ink-jet printing process, especially in the case of large-area recording, such as images, and to guarantee the use of a recording material produced in this way even in ink-jet printers with exacting requirements on the recording material to be used.
To improve the recording quality, especially the permanence of the recording on heat-sensitive recording materials, various proposals have also been made for coatings of the reverse side.
DE 197 48 258 A1 discloses a reverse-side coating, which, in a heat-sensitive recording material, produces a barrier effect against substances used in offset printing and flexographic printing, against organic solvents, and against oils, fats, and softeners. This is intended to prevent discoloration of the heat-sensitive recording coating applied to the front side and to prevent the associated unrecognizability of the typeface produced on the recording coating. This document also cites the following prior-art documents, which are discussed in detail below: DE-C-32 07 071, U.S. Pat. No. 4,593,298, EP-B-0 171 810, EP-A-0 518 552, and DE-A-37 20 171. These documents pertain to heat-sensitive recording materials with reverse-side preparations or coatings, the purpose of which is to improve water resistance and the permanence of the recorded image, to achieve a barrier effect, or to reduce crinkling and curling.
For example, in accordance with DE-C-32 07 071, the reverse-side coating is designed to prevent the penetration of softeners into heat-sensitive recording material used for labels on plastic packages.
The goal of U.S. Pat. No. 4,593,298 is to make available a heat-sensitive recording paper that runs well in thermal printers, shows the least possible curling and is highly resistant to water. The last characteristic is necessary to prevent blocking of rolled recording paper at high levels of atmospheric humidity. This document proposes a heat-sensitive paper with a protective coating applied over the recording coating and with a reverse-side coating that contains a styrene-maleic anhydride copolymer. To guarantee optimum running properties, the document proposes a Bekk smoothness of 30-500 s, and preferably 80-150 s, for the reverse side of the previously known recording paper. It provides no information about Cobb values of the recording coating and especially of the reverse side of the recording paper.
EP-B-0 171 810 describes a heat-sensitive recording material with superior durability and permanence that provides a clear image and high dynamic image density. To this end, the document provides for a reverse side coating that contains a polyurethane-based emulsion binder in addition to a protective coating that covers the heat-sensitive recording coating. Whereas no values are given for the smoothness and water-absorption capacity of the reverse-side coating, examples of smoothness values for the upper coating are specified as 200-300 s and 600-1,000 s, although the measuring method is not specified.
EP-A-0 518 552 provides for a reverse-side coating of a substrate that contains a thermoplastic resin in a heat-sensitive recording material. Substrates that are considered are synthetic papers and especially plastic foils. This is designed to limit potential shrinkage to 1-2%.
DE-A-37 20 171 describes a heat-sensitive recording material to be used for labels. The reverse side of this material is provided with an adhesive coating, whose water-absorption capacity and smoothness are not disclosed.
To limit swelling of the paper substrate by ink penetration of a recording material for ink-jet printing, which possibly has several superimposed coatings that are receptive to ink, EP 0 893 271 A1 provides for a water-repellent coating that contains an inorganic filler and a synthetic binder, which is formed at least on the side of the paper substrate and serves as the substrate for the recording coating. The water-repellent coating is formed with a maximum weight per unit area of 7 g/m2, and paper provided with this coating has a maximum Cobb30 value of 5 g/m2 in accordance with ISO 535 (E1991).
Although the previously known recording materials have proven effective in a wide range of applications, new areas of application are continually developing, and at the same time quality requirements are increasing, so that it is necessary continually to find new solutions to meet these evolving requirements. In particular, heat-sensitive recording materials and papers for ink-jet printing are being used to an increasing extent to produce tickets that have a so-called preprint on the reverse side. These preprints may contain, for example, functional pigments incorporated in the printing ink, especially magnetizable particles with a high perecntage of, for example, iron powder or magnetite.
Extensive series of tests with heat-sensitive recording materials that have these types of barrier coatings on the reverse side have shown that the printability of the reverse side of previously known recording papers has not yet been optimized. Therefore, previously known barrier coatings are still not suitable for different applications in all cases. In particular, the offset printability and the perfect functioning of magnetizable pigments applied by offset printing are not yet ensured with a sufficiently high degree of certainty. It was found that the strength of the magnetic flux that emanates from magnetizable particles is not sufficient if the particles penetrate the paper along with the printing ink and are no longer present on the surface of the reverse side of the paper. As a result, data is incompletely or incorrectly recorded in the preprints by a magnetic head.
A previously used method for preventing excessively deep penetration of the reverse side of recording materials by printing ink and by the functional pigments incorporated in the ink is to underlay the printing ink with a primer coat, i.e., a paint-like coating of printing ink, to seal the paper surface within the printing process. However, this method has the considerable disadvantage that a freely available printing unit is required during the printing process, which is often not the case, especially when multicolor printing is being done.